Spectroscopy systems are typically used to measure radiation intensity as a function of wavelength. Various types of spectroscopic systems exist including transmission spectroscopy, absorbance spectroscopy and reflection spectroscopy. When making optical measurements, particularly when trying to detect low values of optical signals, one has to take into account of the presence of shot noise. Shot noise is a minimum noise level present in standard optical measurements that arises from the particle nature of light. Even with a perfect detector and a perfectly coherent laser source, shot noise still exists because the laser source has a poissonian variance in the number of photons in a beam incident upon the detector at any given time. This variation gives rise to shot noise. Shot noise is particularly problematic when trying to detect low optical signals because such signals are indistinguishable at the detector from the shot noise level. For example, this can be an issue when measuring the absorption spectra of a medium.
Different methods of performing spectroscopy exist. Some techniques use monochromators which selectively output different wavelengths of monochromatic light from a light input covering a broad range of wavelengths. One spectroscopy technique is bi-photon spectroscopy which detects entangled signal and idler photon pairs generated from a nonlinear optical process.
Bi-photon spectroscopy has been demonstrated by Scarcelli et al, “Remote spectral measurement using entangled photons”, Applied Physics Letters 83, 5560 (2003). This paper describes a remote spectrometer using Spontaneous Parametric Down Conversion (SPDC) and a remote scanning monochromator to analyse a range of infrared wavelengths. The signal photon was sent to a remote location passing through an optical element whose spectral function was to be measured. The idler photon passed through the monochromator. The signal and idler photons were detected by photon counting detectors.
Atushji Yabushita et al. “Spectroscopy by frequency-entangled photon”, Physical Review A 69, 013806 (2004) describes generating frequency-non degenerate photon pairs by SPDC, separating the signal and idler photons using a polarising beam splitter and diffracting the vertically polarised signal photons using a grating. The grating was rotatable to allow the apparatus to scan through different wavelengths. The horizontally polarised photons were directed through a partially absorptive sample. The signal and idler photons were collimated into optical fibres by lenses and then detected by a set of two single photon counting modules.
A. A. Kalachev et al. “Biphoton spectroscopy in a strongly nondegenerate regime of SPDC”, Laser Phys. Lett. 5, No. 8, 600-602 (2008) describes generating degenerate biphotons by SPDC using type 1 collinear phase matching, separating the signal and idler photons with a beam splitter, passing the signal photons through a sample whilst sending the idler photons to a monochromator. Both signal and idler photons were detected using separate single photon counting modules.
A. A. Kalachev et al. “Biphoton spectroscopy of YAG:Er3+ crystal” Laser Phys. Let. 4, No. 10, 722-725 (2007) describes generating photon pairs using a LiIO3 crystal cut for type I degenerate collinear phase matching, separating the photon pairs using a beam splitter, passing the signal photons through the sample and sending the idler photons to a monochromator. Both photons were detected using detectors.
The use of rotatable gratings and monchromators makes the above experiments bulky and introduces extra losses into the system, which in turn increases the noise level of the spectroscopy apparatus.
Andreas Jechow at al. “High brightness, tuneable biphoton source at 976 nm for quantum spectroscopy”, Optics express 13439, 18 Aug. 2008/Vol. 16, no. 17, describes a biphoton source using SPDC to generate photon pairs. The signal and idler photons were passed through a 50/:50 beam splitter before being detected by avalanche photodiodes. The use of a standard 50:50 beam sputter prior to the photons being detected by photodiodes splits the signal and idler photons non-deterministically. This set-up cannot provide sub-shot noise performance because the noise of the bi-photon source, when being detected in this manner, is not sub-poissonian.
None of the above documents report sub shot noise performance.